Project Summary: Bacterial pathogens in water pose a large world-wide threat to public health, especially for the population with weak immune system. While current disinfection strategies aim at mitigating the presence of pathogens, they do not completely eradicate the growth of microorganisms. This raises a question as to what happens to the surviving microorganisms after different disinfection processes. It is still unclear that if these surviving microorganisms are the same, less or more hazardous than the ones before disinfection. The answer to that question has a significant impact on microbial safety. As regards effective disinfection, what really matters is reduction of pathogenicity instead of the concentration of microorganisms. The long-term goal is to gain a better understanding on pathogenicity and susceptibility to antibiotics of surviving microorganisms after disinfection and elucidate the role of regulatory defense systems of surviving microorganisms. The proposed research project seeks to reveal the characteristic variations of bacteria before and after different disinfection processes by conducting systematic study to provide guidance regarding disinfection strategies. The primary objectives of this proposal will be achieved by addressing the following specific aims: Specific Aim 1. Investigate microbial community structure and functions in raw and treated water after various disinfections. Specific Aim 2. Determine the virulence and antibiotic resistance of surviving bacteria after disinfections. Specific Aim 3. Elucidate the role of disinfectants in horizontal gene transfer of antibiotic resistance genes and virulence factors. Achieving the above aims will provide fundamental knowledge on the characteristics of surviving bacteria after disinfections which is vital to public health. The enhanced knowledge will facilitate the development of novel disinfection strategies to reduce the pathogenicity of surviving bacteria and minimize the spread of bacteria with virulence and antibiotic resistance into water. These studies will also benefit the future R01 grant, where we will explore the correlations between disinfectant strategies and pathogenicity of surviving microorganisms and elucidate the role of regulatory defense systems of surviving microorganisms.